To lay supply lines, such as water pipes or cables beneath streets or embankments or other building structures or obstacles, without digging trenches, it is known to drive a boring rod into the ground by means of a percussion machine arranged in an excavation and to statically push the boring rod and set it in rotation by means of a pneumatically or hydraulically driven drive unit likewise arranged in the excavation on a mounting. Furthermore, instead of allowing the percussion machine to act on the rear end of the boring rod, it is known to arrange at the tip of the boring rod a ram boring machine having a percussion piston which can reciprocate in a tubular housing of the machine. In both cases the generally necessary starting excavation is required to be of a minimum length which corresponds to the length of the boring rod part to be driven-in, together with the length of the percussion machine. Owing to the often very restricted availability of space in the (starting) excavations it is still desirable, even when making bore holes in the earth by displacement of the earth, that the ram boring machines used for this purpose, which work their way into the earth under the impacts of the percussion piston, have a short overall length.
Both in the case of self-propelled ram boring machines which make bore holes in the earth according to the earth displacement principle and machines which only deliver impacts--which latter, in contrast to the earth displacement machines, need not necessarily be provided with a reversing mechanism for ensuring backward running of the machine--it is known to use for the reciprocating movement of the percussion piston, and for changing the running direction of the ram boring machine, a control mechanism which works on the plunger-piston-principle. This method of construction requires a certain machine length, because a control piston is accommodated by an axial bore in a tubular section of the percussion piston forming an inner cylinder or pressure chamber. The forward and backward movement of the percussion piston, as well as the prevailing running direction of the ram boring machine, can be set, for example, by means of a fixed control sleeve with control openings, which controls the covering of radial control openings located in the percussion piston and which is connected to a bearing tube, and a control sleeve with corresponding control openings which is rotatably mounted in the bearing tube.
In the case of a ram boring machine disclosed in German patent 23 40 751 having a control mechanism for the forward and backward running, a control sleeve is held in both operating positions against a guide ring provided with venting bores, by means of stops. The control sleeve is located on an outer tube and is fixed in position; an inner tube provided with radial openings is rotated. This tube provides that either a front or rear control bush comes fully into play and controls the direction of movement of the piston. For reversal, the rotation locking mechanism must be released by means of a rope or cable. Apart from the plunger-piston-principle, which increases the structural length, with the method of construction of the known ram boring machines only small passages, such as in particular minimal annular slots between the piston and the machine housing, are left for the exhaust air that flows to the rear end of the machine, which can result in a back-up of flow and a reduction in the power of the ram boring machine. Finally, the compressed air for providing forward movement of the percussion piston can only act on the inner piston bore surface, namely the inner pressure chamber of the percussion piston.
A compact ram boring machine is known from U.S. Pat. No. 4,070,948. Simply in order to be able to provide the connections for fluid flow between the pressure medium supply and the pressure chambers necessary for reversing the percussion piston and for switching the direction of running of the ram boring machine, the compact construction necessarily requires a multi-stepped percussion piston and/or the bore of the housing to be turned out. The pressure medium supplied from the rear end of the housing acts, in accordance with the plunger-piston-principle, on the front face of the smallest diameter step of the percussion piston, which step part is piston rod like with an axial air bore and enters the end screw fitting, in which a pressure chamber bounded forwardly by the front face forms upon the forward movement of the percussion piston. When striking the front face, the pressure medium thus also simultaneously enters the axial, restricted-length air bore, the end of which is provided with at least one radial distributor bore which leads the pressure medium into the pressure chamber located in front of the percussion piston. Aside from the great technical/manufacturing effort involved--no smooth housing bore; numerous piston bores or passages--only a small area of the percussion piston, namely the smallest step section, can be acted on by the pressure medium in this known machine also.